Development of Atomically Dispersed ReOx/SiO2 Catalysts for Selective Methanol Carbonylation to Acetic Acid

Acetic acid (AA) is an important bulk commodity chemical produced primarily via methanol carbonylation using homogeneous organometallic catalysts in liquid phase reactors with corrosive halide-based co-catalysts. Here we demonstrate a heterogeneous catalyst based on atomically dispersed rhenium (ReO₄) active sites on an inert support (SiO₂) for stable halide-free, gas phase carbonylation of methanol to AA. Atomically-dispersed and stable ReO₄ species were deposited on mesoporous high surface area (700 m²/g) inert SiO₂ using triethanolamine as a dispersion promoter and characterized using scanning transmission electron microscopy (STEM), UV-Vis spectroscopy and X-Ray absorption spectroscopy (XAS). While ReO_x clusters (formed > 10 wt%) were primarily selective for DME formation, atomically dispersed ReO₄ species (formed below 10 wt%) were found to exhibit stable (for 60 hours) > 93% selectivity to AA with single pass conversion > 60%. Kinetic analysis, in-situ FTIR and in-situ XAS suggest that the reaction mechanism involves methanol activation on Lewis acidic ReO₄ sites, followed by direct CO insertion into the terminal methyl species. Further, by introducing ~0.2 wt% of atomically dispersed Rh to 10 wt% atomically dispersed ReO₄ on SiO₂, > 96% selectivity toward AA production at volumetric reaction rates comparable to homogeneous processes were observed. This work introduces a new class of promising heterogenous catalysts based on atomically dispersed ReO₄ on inert supports for alcohol carbonylation.